Anode slimes obtained from the tanks of a copper electrorefinery are processed to recover various metal values contained therein. In particular the anode slimes contain varying amounts of selenium and in fact such slimes constitute the largest source of that element. In addition to selenium, however, the slimes also contain copper and tellurium, and depending on the particular copper electrorefinery and the source ore involved they may also contain nickel, silver, gold, lead, antimony, bismuth, arsenic and iron.
Processes aimed at recovering selenium from anode slimes most commonly involve, as a first step, treating the slimes with sulfuric acid to convert such metals as copper, nickel and iron to soluble sulfates. The sulfation process which is usually carried out at elevated temperatures solubilizes most of the tellurium present while leaving most of the selenium in an oxidic form so that a roasting operation following the sulfation yields selenium dioxide vapor. Apart from environmental problems associated with the vaporization step, such a sulfation which may be carried out in kettles heated to about 330.degree. C. to yield a paste which is pan-roasted at about 400.degree. C. is limited to implementation as a batch operation.
An alternative procedure aimed at avoiding the vaporization of selenium dioxide is described in a paper entitled: "Treatment of Anode Slimes at the INCO Copper Refinery" presented by R. K. Monohan and F. Loewen to the CIM Annual Conference 1972, Halifax, Nova Scotia. The procedure described comprises reacting the slimes with a sufficiently large amount of sulfuric acid to retain fluidity in a continuously stirred tank maintained at 215.degree. C. This results in solubilization of the copper and nickel without vaporization of selenium and the process is amenable to a continuous mode of operation. An inherent difficulty is presented by the need to use a large excess of acid to maintain the fluidity of the charge in the tank. A typical amount of acid needed would be about 400% of the weight of the moist slimes, which is equivalent to about 500% of the weight of slime solids. The operating costs of the process are therefore adversely affected by the cost of the acid needed as well as the subsequent acid-bleeding operation which has to be undertaken if the solution is to be recycled to the copper refinery.